The present invention relates to an optical system for focusing by moving a part of the optical members, which system is particularly capable of focusing without changing the size of the pupil of the optical system.
Generally, the entire lens or a part of the elements are moved along the direction of the optical axis for focusing the object by means of the photographing lens, whereby in order to focus the eye fundus with an ophthalmic investigation instrument such an as eye fundus camera or ophthalmic refractometer, the photographing film or a part of the image forming optical system is moved.
Below, the problems accompanied with the focusing will be explained in accordance with an ophthalmic investigation instrument having many such difficulties.
FIGS. 6 and 7 respectively show a conventional focusing method for ophthalmic instruments. In FIG. 6, E is the eye to be inspected, Ef is the eye fundus, P is the pupil, L.sub.1 is the objective lens and L.sub.2 is the imaging lens, whereby the image lens L.sub.2 serves to form the image of the objective lens L.sub.1 on the film again. M is a quick return mirror for picking up the view finder light beam. A is the photographing aperture stop, arranged at a position conjugate with the pupil P with reference to the objective lens L.sub.1 and corresponding to the focal plane at the object side, of the image forming lens L.sub.2. When in this case the image in the air, of the object is focused by moving the film F and the mirror M at the same time along the direction of the optical axis and observing the image through the eye piece of the view finder, the dimension of the pupil through which the light beam, namely, the image of the aperture as seen from the film side does not change, while the eye piece is moved forwards or backwards during the focusing operation. Thus, the observer has to move his face forwards or backwards in order to follow the eye piece, which is inconvenient.
In FIG. 7, L.sub.3 is the focusing lens, which is moved along the direction of the optical axis for focusing. In the case of the present instrument, it is possible to carry out focusing without moving the face, while along with the movement of the lens L.sub.3, the position of the pupil through which the light beam changes and at the same time the dimension of the pupil also changes so that the magnification factor as well as the light amount change.